you just take the result parse tree, search for "WHERE" node and modify the tree (add/remove children) the way you want. Look at the code that 'displays' the tree in the treeView in Grammar Explorer - it is very simple to iterate; and to modify
it is not a problem - all child lists are open for add/remove

I don't think there're any examples or tutorials, but basically, you're free to do what you want, construct nodes with terminals/nonterminals inside, using available constructors. Once parsing is done, Irony parser produces the tree and you can do whatever
you want there.